Fresh questions have been raised over the ability of Andrew Flintoff to get through a gruelling five-match Ashes series after he suffered what the England camp described yesterday as "slight discomfort" to his left ankle, which was operated on in July for the second time in 18 months.

England's captain bowled only four overs at considerably less than full tilt on the fourth day of the second Test, all first thing in the morning, before leaving the field later. The England coach, Duncan Fletcher, insisted Flintoff's decision to come off was only a precaution, but the Australians believe the injury might be more serious: a dressing-room source last night described his chances of playing a pain-free role in the rest of the series as "cooked".

Despite that, Fletcher said the medical staff were sufficiently confident not to demand a scan. But the spectre of yet more ankle trouble for England's crucial figure is the last thing he wanted to be discussing so early in the series. If there is any good news, it is that the third Test in Perth is not until December 14; the bad news is that the potentially vital Tests at Melbourne, starting on Boxing Day, and Sydney come in quick succession.

Flintoff earned the everlasting respect of the Australians in 2005 and his importance with the ball was underlined at Brisbane, where he took four for 99 in a first-innings total of 602 for nine. If his effectiveness is reduced over the next three Tests, England will struggle to retain the Ashes.

Fletcher said: "He's had quite a bit of bowling, but there are no worries at the moment. We'll have to wait and see over the next couple of days. He's had this problem in the past but the medical staff are pretty confident. Hopefully this is just a result of having overbowled in back-to-back Tests."

Whether Flintoff really has been overbowled at Brisbane and Adelaide is a moot point. After Australia's first innings here his tally of overs for the series was 61, fewer than any England seamer other than Jimmy Anderson, who had bowled only 59.3 overs largely because it has been difficult to trust him. With another sluggish pitch expected in Perth, Flintoff cannot afford his ankle to be playing on his mind.

Bill Ribbans, the orthopaedic surgeon who along with John Hodgkinson operated on Flintoff's ankle this summer, explained the England captain is far from the only victim, saying it is "rapidly becoming a fast bowler's problem. McGrath had the problem and I understand he was off for a year recovering. Brett Lee has had it, as has Shaun Pollock and Sajid Mahmood."

"The type of injury is at the back of the ankle, it's not in the ankle joint itself. It is called a posterior impingement of the ankle, caused by a number of different problems. You can divide them into two types - a bone problem and a soft-tissue problem. The bone problem can be either because people are born with a particularly large piece of bone at the back of the ankle or they can be born with a separate piece of bone, called an os trigonum - 10% of us have this separate bone and it doesn't cause us any problem at all. But certain activities it can aggravate it.

"But what seems to have happened with all the fast bowlers is that it's the impact from coming down on the front foot. They have to put their foot out in front and it almost has a nutcracker effect at the back of the ankle. The back of the shin bangs into this extra bit of bone and it causes inflammation. The soft tissue part can be inflammation of the tendons or of the 'capsule', which is the back of the lining of the joint. Sometimes you can get a combination of both - soft tissue and bone."

None of his attempts to ease concerns will stop the speculation. Flintoff first had surgery on his left ankle in January 2005 to remove a bone spur. He then missed the four-Test series against Pakistan last summer after a second operation, with Ribbans, before a punishing 12-week fitness regime. When he delivered his first ball in Brisbane, he had bowled only 52 competitive overs since breaking down in a match in July.

Flintoff admitted in August his ankle might niggle him for the rest of his career - "There is a possibility I'll have to continue with my rehab until I retire. If that's the case I'll just have to grin and bear it, but it would be a small price to pay for continuing to play cricket for a living."